1. Technological Field
The invention relates to hollow membranes intended for the treatment of fluids (liquids and/or gases) with a view to separating from them one or more constituents by absorption, adsorption and/or transfer phenomena through a membrane produced in a material having properties specific to one or more of the treated fluids. It is also applicable to the transfer of material and/or heat between two fluids separated by said membrane.
The invention also relates to treatment modules for fluids that include such membranes. These modules can be used in various fields, for example, to wash acidic gases for the preparation of synthesis gases and to combat environmental pollution by purifying the gases from a furnace or by treating aqueous effluent.
The invention is also applicable to biological processes such as fermentation, the manufacture of proteins, biological oxidation processes as well to medical equipment such as blood oxygenators and artificial kidneys.
2. State of the Prior Technology
The membranes used for the treatment of fluids up to now are, either flat membranes or membranes in the form of hollow fibers.
For the latter, the possibility has been studied of producing them in the form of hollow fibers of small length and diameter as described in document 1:WO-A-95/00238. Limiting the length of the hollow fibers allows one, in particular to limit the pressure drop of the fluid circulating in them as is the case in natural capillary membranes such as those in the human lung. In effect, in these natural systems, the capillaries that supply the blood have an internal diameter as low as 7 xcexcm, but they have low flow resistance because of their extremely short length of about 100 xcexcm. This is the reason why natural systems are so efficient for mass transfer.
Document 1 illustrates a membrane panel of self-supporting hollow fibers comprising two base layers of a textile material encapsulated in a non-permeable material, and a multiplicity of hollow fibers of permeable material that extend between the two layers already mentioned. Hence, in this membrane with hollow fibers, the support layers have no particular property whatsoever in relation to the fluid to be treated since they are made of a non-permeable material.
Document 2: U.S. Pat. No. 4,959,152 describes an assembly of hollow fibers comprising a plurality of stacked discs in which the hollow fibers are arranged horizontally so that a fluid circulates in parallel in all of the discs. These fibers are shorter than in traditional devices but they still have a large length compared with the size found in natural systems such as the human lung.
Document 3:U.S. Pat. No. 5,104,535 describes an assembly of hollow fibers mounted between two end supports and assembled one above the other to form modules which are arranged side by side within a treatment enclosure. As in the preceding document, the hollow fibers still have lengths which are large compared with that found in natural systems used for material transfer.
With the techniques described above, the following problems have to be faced. Because of the large thickness of the membranes in the form of hollow fibers, there is always a requirement to make the surface of the pores of said membranes hydrophobic by using rather complex methods which in addition are not sufficiently reliable. So as to prevent the passage of a liquid containing the component to be transferred, through the pores of said membranes it is always necessary to make a precise adjustment of the differential pressure on both sides of said membranes. Using membranes in the form of hollow fibers having a length of the order of one meter and a length to internal diameter ratio of the order of 2000, a pressure difference between the inlet and the outlet of said fibers is obtained which is too great. If one tries to reduce the thickness of the membrane in the form of hollow fibers, the reliability of the device is reduced since the probability of rupturing said membranes is increased. Furthermore, the absence of rigidity in the hollow fibers means that these fibers have a tendency to stick to one another under the action of the fluid flows thereby causing the hydrodynamics flow conditions of the fluids to deteriorate.
In natural capillary systems like a lung, an intestine or a kidney, there is a vast number of more or less short capillaries on the surface being used for material transfer. These are the alveoli in the lungs or the epithelium, the villosities and micro-villosities in the intestines and finally the glomerulic capillaries in the kidneys which comprise fine capillaries with a length to diameter ratio of between about 10 and about 30. Numbering about 5xc3x97108, the pulmonary alveoli represent a surface area of about 200 m2. Grouped in little clusters, the alveoli are formed from cells with a very thin wall. The transfer of gases (oxygen and carbon dioxide) is carried out through the walls of these alveoli cells. The mass of blood which passes in 24 hours in the lungs is estimated to be 10 m3.
Thanks to the micro-villosities found on the external surface of the wall of biological cells making up the intestinal epithelium, the geometric absorption surface area of each of these cells increases several hundred times. These intestinal villosities carry out continuous to and from movements in the liquid pulp resulting from digestion. The passage of digested foodstuff into the blood and the lymph is encouraged thanks to the turbulence in the liquid medium. In the case of the lungs, a system of capillaries is observed the diameter of which progressively decreases along the path of the aspirated air from the trachea towards the alveoli, the number of capillaries increasing in the same direction. This is why natural capillary systems are so efficient for the transfer of material from a surrounding medium into the blood, through the walls of capillaries formed by a biological route.
Material transfer in capillaries of small diameter takes place under a laminar fluid flow regime. So as to make the transfer more intensive under specific conditions it is necessary to have short capillaries of small diameter and a small intercapillary distance, on the one hand and capillaries with a thin wall on the other hand. By using hollow fibers for this purpose, the material transfer step in a gas-liquid system is limited by the rate of diffusion in the liquid phase, and the overall rate of the transfer process is proportional to the total surface area of the membrane despite the porosity of the membrane (or the wall of the hollow fibers).
When using hollow fibers, having the properties described above, one makes a distinction between two operating regimes for the membrane: a wetted membrane and a non-wetted membrane. Obtaining one or the other regime depends on the pressure used and the interaction between membrane and liquid. The strength of a membrane, the pores of which are full of a liquid phase (wetted regime) is much greater than that of a membrane the pores of which are full of a gaseous phase.
This invention proposes a resolution of the problems described above by means of a hollow membrane, with capillary tubes, the structure of which is much closer to that of natural biological systems.
To this end, the invention proposes a hollow membrane comprising two support layers arranged one above the other forming a space between them and a plurality of capillary tubes arranged between the support layers forming capillary channels for the flow of a first fluid, the space between the capillary tubes forming an internal cavity for the circulation of a second fluid around the capillary tubes, and the two support layers and the capillary tubes being made of an organic polymer.
This particular structure for the hollow membrane of the invention offers a number of advantages. In effect, the capillaries formed between the two support layers can have the following characteristics:
a very small length, for example from 1 to 1000 micrometers (xcexcm) preferably from 3 to 200 xcexcm and more preferably from 5 to 60 xcexcm,
a very small internal diameter, for example from 0.02 to 50 xcexcm, preferably from 0.1 to 10 xcexcm,
a very small wall thickness, for example, from 0.01 to 10 xcexcm, preferably from 0.1 to 3 xcexcm, and
a very high number of capillary tubes per unit surface area, for example from 104 to 8xc3x97109 capillary tubes/cm2, preferably from 105 to 5xc3x97108 capillary tubes/cm2.
Hence a structure is provided that has characteristics close to those of natural systems such as the lung, the kidney and the intestine.
Furthermore, the hollow membrane of the invention has not only an exchange surface at the capillary tubes but also at the two support layers which are made of the same material as the capillary tubes.
According to the invention, the hollow membrane is produced preferably in an organic polymer capable of being obtained by chemical or electrochemical oxidation of a precursor monomer.
In particular, such polymers can be heterocyclic polymers or polyacetylene. As an example of heterocyclic polymers one could mention the polypyrroles the polyanilines and the polythiophenes.
It should be made clear that by polypyrrole one understands not only the polymers obtained from pyrrole but also those obtained from pyrrole derivatives. The same applies for polyanilines and polythiophenes.
In the hollow membrane of the invention, the capillary tubes are preferably arranged substantially perpendicular to the two support layers and/or along directions that make angles of, at the most 45xc2x0 with the perpendicular to the support layers.
The hollow membrane of the invention can be used to transfer a component from a first fluid fed under pressure into the capillary tubes of the hollow membrane to another fluid flowing in the internal cavity of the hollow membrane. Hence the two fluids which are participating in the transfer process of the component in question are separated in the hollow membrane by a separation layer that comprises on the one hand the walls of the capillary tubes and on the other hand the two support layers.
Another subject of this invention is a method of manufacturing a hollow membrane having the characteristics given above.
This method comprises the following steps
a) forming on the external surfaces and in the pores of a membrane matrix, that includes open rectilinear pores arranged between its two external surfaces, a coating of organic polymer by in situ polymerization of a precursor monomer of the polymer, and
b) then removing the material forming the membrane matrix by destruction in a selective reactant which does not affect the polymer in order to form the internal cavity of said hollow membrane.
In this method, one starts with a membrane matrix that includes rectilinear pores having dimensions slightly greater than those of the capillary tubes to be produced and a thickness that corresponds to the length of the capillaries to be produced.
This membrane matrix can be made of a polymeric material or an inorganic material. Preferably, the rectilinear pores have been created in this membrane matrix by irradiation with a beam of heavy ions followed by dissolution of the material in the tracks made by the ions and/or around them. A technique of this type is described in the following documents:
Flerov G. N. Synthese des elements superlourds et application des methodes de physiques nucleaire dans les domaines voisins. Vestnik de l"" academie des sciences de l""URSS, 1984 no. 4, p. 35-48 (in Russian).
Apel, P. Yu, Kuznetsow, V. I., Zhitariuk, N. I. and Orelovich, O. L. (1985) Nuclear ultrafilters. Colloid Journal of the USSR, 47, 1-5 (in English).
The polymeric materials capable of being used can be, for example, polycarbonates, polyethylene terephthalate, polyimides or polyvinylidene fluoride. With such polymeric materials, one can use as the selective reactant in step b) of the method, bases and inorganic acids or potassium permanganate in the case of polyvinylidene fluoride.
The inorganic materials capable of being used to form the membrane matrix can be, for example, aluminum oxide or mica. In the case of aluminum oxide, the reactant used in step b) can be an inorganic base or an inorganic acid. In the case of mica, hydrofluoric acid is preferably used as a reactant in this step b).
The membrane matrices that can be used in the invention having a thickness of from 1 to 1000 xcexcm, preferably from 3 to 200 xcexcm and more preferably from 5 to 60 xcexcm, a pore diameter of from 0.02 to 50 xcexcm, preferably from 0.1 to 10 xcexcm and a pore density of 104 to 8xc3x97109/cm2, preferably from 105 to 5xc3x97108 cmxe2x88x922 are sold by the following companies: Costar, Millipore, Osmonics (United States of America), What man (Belgium, England) and Centre de Physique Appliquee de l"" Institut Uni des Recherches Nucleaires (Russia).
Mineral membranes that can be used as a membrane matrix are sold under the trade mark Anopore(copyright) by the company Whatman and they have a pore diameter of from 0.1 to 0.2 xcexcm, a thickness of about 60 xcexcm and a porosity of from 40 to 60%.
According to the invention, a coating of organic polymer is deposited on these membrane matrices, not only on the external surfaces of the membrane matrix, but also in the pores of the membrane, by in situ polymerization of a monomer precursor of the polymer.
This polymerization can be carried out chemically or electrochemically and is applied to monomer precursors chosen from the group comprising the following heterocyclic compounds: pyrrole, aniline, thiophene and their derivatives as well as acetylene. The polymerization of the monomer precursor can be initiated in an aqueous phase or an organic phase using polar solvents such as acetonitrile and propylene carbonate or a mixture of an organic solvent (an alcohol for example) and water, by an oxidizing agent chosen from the group comprising ferric (Fe3+) salts, tetra-alkylammonium periodates, perbromates or perchlorates or the periodates, perbromates or perchlorates of other cations, or salts containing the persulfate anion. The polymerization is carried out preferably by bringing one face of the membrane matrix into contact with a solution of the monomer precursor and the other face of the membrane into contact with the oxidizing agent.
So as to obtain a polymer layer, that is to say a separation layer, having different pore sizes, different degrees of wetting by aqueous solutions or organic solvents, and/or different electro-conductivity properties, rigidity, porosity and flexibility, one can:
a) add to the solution that already contains the oxidizing agent, a doping agent chosen, in general, from among the Lewis acids, preferably from among the compounds that have an anion of the (Rxe2x80x94SO3xe2x88x92) type where R is an alkyl, aryl or alkylaryl group, such as the alkylbenzene sulfonic acids, alkyltoluene sulfonic acids or salts of said acids;
b) carry out a co-polymerization by grafting onto said separation layer unsaturated compounds chosen from the group comprising acrylic acid, vinyl pyrrolidone, vinyl pyridine, acrylamide and their derivatives
c) treating said separation layer in a plasma or with radiation from an excimer (excited dimer) laser with or without injection of organic molecules (benzene and its derivatives, alkenes); or
d) treating said polymer with mineral alkalis such as sodium or potassium hydroxide.
The doping or de-doping process described above can lead to a change in the sike of the pores in the layer of deposited polymer, that is to say in the separation layer. By doping this layer. Preferably during the polymerization, one arrives at a separation layer without any pore at all. By subsequently carrying out a de-doping of the separation layer by treatment in basic media, the ions (molecules) of doping agent are removed from said layer and they are replaced by another doping agent having a smaller size. The size of said pores depends on the size of the ions or molecules of doping agent that are removed: the bigger they are, the bigger the pores in the separation layer are.
The treatments described above can be carried out at the time of the polymerization of the monomer precursor, or as a complementary treatment after the formation of the polymer layer.
The thickness of the polymer layer deposited on the surfaces and in the pores of the membrane matrix depends on the polymerization conditions, the concentration of reactants in the polymerization solutions and the duration of the polymerization. Generally, one operates at a temperature of from xe2x88x9235xc2x0 to +30xc2x0 C. Generally the thickness of the polymer layer deposited is greater on the surfaces of the membrane than on the walls of the rectilinear pores, or capillary tubes. In order subsequently to carry out step b) of destroying the material forming the membrane matrix, it suffices to make an opening in the deposited polymer layer and to cause a reactant to enter that is capable of destroying the material forming the membrane matrix while leaving the layer of deposited polymer intact.
After this operation, one may, if necessary, subject the polymer layer to the doping and de-doping treatments described above.
A further subject of the invention is a module for the treatment of a fluid comprising at least one hollow membrane such as that defined above arranged within a sealed enclosure in such a way as to provide, between two adjacent hollow membranes and between each hollow membrane and an adjacent side wall of the enclosure, spaces for the circulation of a first fluid uniquely in communication with the inside of the capillary tubes of the hollow membrane or membranes, means of circulating said first fluid in the capillary tubes of the hollow membranes by introducing it into one of said circulation spaces and by collecting it in another of said circulation spaces and means of circulating at least one second fluid in the internal cavity or cavities of the hollow membrane or membranes.
Preferably in this module, the spaces for circulation of the first fluid are filled with packing that allows turbulence to be generated in the first fluid.
This packing can, in particular, be in the form of a porous material, the pores of which have dimensions that are greater than the diameter of the capillary tubes.
By way of example, circulation spaces for the first fluid can be produced in the form of porous panels, that have, for example, a pore dimension of from 0.1 to 200 xcexcm, preferably from 5 to 150 xcexcm and a thickness of from 10 to 1000 xcexcm, preferably from 20 to 200 xcexcm. The pore dimensions of this porous material are preferably such that the ratio of the pore size to the diameter of the capillary tubes is from 5 to 200, preferably from 10 to 200 and more preferably from 10 to 50.
In the module of the invention, these porous elements which are arranged on either side of the hollow membrane or membranes are intended to distribute the fluid to the capillary tubes of the hollow membrane or membranes and to generate turbulence in said fluid by forming hydraulic eddies in it thanks to a rise in the Reynolds number. This allows one to intensify the material transfer and or the heat transfer in the fluid before it enters the capillary tubes of the hollow membrane.
In order to improve the separations carried out in the hollow membrane, one may also partially fill the pores of the porous material with a compound chosen from among catalysts, enzymes and sorbents that are insoluble in said first fluid in such a way as to carry out catalytic or other reactions in the first fluid before its entry into the capillary tubes of the hollow membrane. Such a reaction can be carried out, for example in order to retain a component and/or fine particles of the first fluid before its treatment in the capillary tubes. A catalytic reaction can also be used to obtain a component subsequently separated in the capillary tubes.
Of course the lining of the pores is carried out in such a fashion that the ratio of the pore dimension of the porous material to the internal diameter of the capillary tubes remains within the range of from 10 to 50 and the size of the catalyst or sorbent particles must be greater than the internal diameter of the capillary tubes.
According to the invention, one may assemble several hollow membranes in such a way as to form a stack that allows the fluid to be treated or the first fluid, to be brought into contact through the walls of the capillaries with a second and possibly a third fluid.
According to a first embodiment of this assembly, the module comprises:
a stack of n hollow membranes and (n+1) panels of porous material alternating with the hollow membranes in such a way that each hollow membrane is positioned between two panels of porous material, these panels forming spaces for the circulation of the first fluid,
means of introducing the first fluid onto the lower or upper face of the stack and of recovering it at the opposite face of this stack,
a chamber for the introduction of the second fluid, arranged on a lateral face of the stack and in communication with the internal cavities of the hollow membranes, and
a chamber for receiving the second fluid arranged on the opposite lateral face of the stack and in communication with the internal cavities of said hollow membranes.
According to a second embodiment of this assembly, the module comprises
n hollow membranes and (n+1) panels of porous material alternating with the hollow membranes in such a way that each hollow membrane is positioned in a stack between two panels of porous material, these panels forming spaces for the circulation of the first fluid, the stack comprising a first series of hollow membranes with an odd number and a second series of hollow membranes with an even number arranged between the membranes with an odd number,
means of introducing the first fluid onto the lower or upper face of the stack and of recovering it at the opposite face of this stack,
a chamber for the introduction of the second fluid, arranged on a first lateral face of the stack and in communication with the internal cavities of the hollow membranes of the first series,
a chamber for receiving the second fluid arranged on the lateral face opposite to said first face and in communication with the internal cavities of said hollow membranes of the first series,
a chamber for the introduction of a third fluid, arranged on the lateral face, called the second lateral face, contiguous with said first lateral face and in communication with the internal cavities of the hollow membranes of the second series,
a chamber for receiving the third fluid arranged on the lateral face opposite to said second face and in communication with the internal cavities of the hollow membranes of the second series.
According to a third embodiment of this assembly, the module comprises:
a stack of n hollow membranes and (n+1) panels of porous material alternating with the hollow membranes in such a way that each hollow membrane is positioned between two panels of porous material, these panels forming spaces for the circulation of the first fluid, the stack comprising a first series of hollow membranes with an odd number and a second series of hollow membranes with an even number arranged between the membranes with an odd number,
means of introducing the first fluid onto the lower or upper face of the stack and of recovering it at the opposite face of this stack,
a chamber for the introduction of a second fluid, arranged on a first lateral face of the stack and in communication with the internal cavities of the hollow membranes of the first series,
a chamber for receiving the second fluid arranged on the lateral face of the stack, called the second face and contiguous with said first face and in communication with the internal cavities of said hollow membranes of the first series,
a chamber for the introduction of a third fluid, arranged on another lateral face of the stack, called the third face, and in communication with the internal cavities of the hollow membranes of the second series,
a chamber for receiving the third fluid arranged on the last lateral face of the stack, called the fourth lateral face, said chamber being in communication with the internal cavities of the hollow membranes of the second series.
Preferably, in these three embodiments of the assembly, the stack is arranged between two rigid grids, whose openings are at least equal to or greater than the pore dimension of the panels of porous material. Furthermore, in these assemblies, one can use hollow membranes, at least one of which has different characteristics with regard to the diameter, the length and/or the quantity of capillary tubes, in order to obtain particular effects.
Preferably, the diameter of the capillary tubes reduces from one hollow membrane to the other in the direction of flow of the first fluid while the number of capillary tubes per unit of surface area of hollow membrane increases from one hollow membrane to the other in the direction of flow of the first fluid.
This arrangement allows one to have a module with characteristics that are closer to those of natural capillary systems.
The modules described above can be produced by a method comprising the following steps
1) Preparing at least one composite membrane by forming, on the external surfaces and in the pores of a membrane matrix comprising rectilinear open pores arranged between its two external surfaces, a coating of organic polymer by in situ polymerization of a monomer precursor of the polymer,
2) Forming, from the composite membrane or membranes and panels of porous material, a stack in which each composite membrane is arranged between two panels of porous material,
3) Forming sealed joints between the composite membranes and the porous panels on the lateral faces of the stack,
4) Making openings in the sealed joint only at the level of the composite membranes, and for each composite membrane only on two different lateral faces of the stack,
5) Introducing through these openings a reactant capable of destroying the material forming the membrane matrix of the composite membranes without affecting the polymer covering the surfaces and the pores of the membrane matrix, in order to obtain a stack of hollow membranes and panels of porous material in which the internal cavities. of the hollow membranes are accessible on two lateral faces of the stack.